I picked up a Mapletree Audio Designs Octal 6 Vacuum Tube Preamp to use with my Carverfest Monoblocks. It is here in Wilkes-Barre with me .Cant wait to try it out when I get home on Thursday. Picture is of the unit which is as depicted on Mapletree's site.

First of all, I wish to thank Richard for the time and effort spent in building and reviewing the MAD Octal 6. Such reviews are a great service to the audiophile consumer. I am pleased that the Octal 6 provided a rewarding kit building experience. I certainly relied on my experiences years ago with Heath and Dynakits when designing the physical assembly procedures and instruction manual. Richard's listening impressions reflect the design intent to a large degree. The choice of carbon film resistors was taken to achieve a fuller, warmer sonic signature than is often obtained with (cheaper) generic metal film types. Both the review and customers' feedback seem to indicate that this objective has been met. It is also interesting that for Richard (and other listeners), the sonic differences in the two active topologies are in the first instance, subtle. Nevertheless, listeners soon develop a preference, which seems about equally split between the two. With the Octal 6, you do have a choice.

Perhaps a word about "tube rolling" would be helpful. The 12SX7GT is considered by many to be the top 12SN7GT type ever made and can be directly substituted. Also, there are metal versions of the 12J5GT and 6R7G which can be tried. Also, if at some point in the future the user wishes to use 6 V versions of the 12J5 and 12SN7, heater rewiring is quite straightforward. For now, the 12 V versions are a much better deal, as the price for NOS 6SN7GTs is out of sight.

Two modifications have been made to the Octal 6 since the review sample was shipped, which have significant impact on the performance. First, an Alps "Black Beauty" center-detent balance control is now included. This special control (which is no longer manufactured) provides zero attenuation at mid-position. This gives the Octal 6 about an 8 dB increase in gain in all modes, which is particularly welcome in the passive mode. The precision and reliability of the control are also welcome. Secondly, the position of the volume and balance controls in the circuit has been reversed to lower the output resistance of the passive path. This can have a significant impact on high frequency response when driving capacitive cable runs to the power amplifier. This is a seldom mentioned problem with passive control "preamplifiers".

Lastly, I am not happy with the fact that Richard heard low level hum when using the Octal 6. The samples I have built as assembled units certainly had no audible hum close to the speaker when used with a power amp with a gain of around 10 and speakers with 92 dB sensitivity. Higher gain power amps or more sensitive speakers will of course effectively amplify any output noise from any preamp to audible levels. With the dc heater supply and the use of shielded cable on all the inputs, every effort was taken to make the Octal 6 a very quiet preamp. If a customer is unsatisfied with any aspect of the Octal 6 kit performance (relative to the published specs), I am happy to have it shipped back for a tuneup at no cost to the customer except shipping.